Catalyst deterioration detecting system and catalyst deterioration detecting method of internal combustion engine

ABSTRACT

A catalyst deterioration detecting system of an internal combustion engine includes a measuring unit that measures the oxygen storage capacity of a catalyst, a detector that detects or estimates the temperature of the catalyst, and a detector that detects poisoning of the catalyst based on the relationship between a change in the catalyst temperature and a change in the oxygen storage capacity corresponding to the change in the catalyst temperature. The change in the oxygen storage capacity corresponding to the change in the catalyst temperature varies depending on the presence or absence of poisoning of the catalyst. Thus, the catalyst deterioration detecting system is able to favorably detect poisoning of the catalyst by utilizing this relationship, and distinguish temporary deterioration due to catalyst poisoning from permanent deterioration. A method of detecting deterioration of the catalyst is also provided.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2007-011247 filed on Jan. 22, 2007, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a catalyst deterioration detecting system that detects deterioration of a catalyst disposed in an exhaust passage of an internal combustion engine, and also relates to a catalyst deterioration detecting method.

2. Description of the Related Art

Generally, a catalyst is disposed in an exhaust passage of an internal combustion engine so as to treat or clean exhaust gas. One type of the catalyst is, for example, a three-way catalyst that has an O₂ storage function of adsorbing and releasing oxygen depending on the air/fuel ratio of exhaust gas entering the catalyst. More specifically, the catalyst adsorbs and stores excessive oxygen present in the exhaust gas when the air/fuel ratio of the exhaust gas entering the catalyst becomes larger than the stoichiometric air/fuel ratio, namely, becomes lean, and releases the stored oxygen when the air/fuel ratio becomes smaller than the stoichiometric ratio, namely, becomes rich. Accordingly, even if the air/fuel ratio of a fuel-air mixture supplied to the engine swings to the rich side or lean side relative to the stoichiometric air/fuel ratio depending on operating conditions during normal operation of the engine, the surface of the catalyst is kept at the stoichiometric air/fuel ratio, and the three-way catalyst having the O₂ storage function is able to convert NOx, HC and CO into harmless substances at the same time. More specifically, when the fuel-air mixture becomes lean, excessive oxygen is adsorbed onto and stored in the catalyst so that NOx is reduced. When the mixture becomes rich, the oxygen stored in the catalyst is released from the catalyst, so as to oxidize HC and CO.

In a conventional system, an air/fuel ratio sensor for detecting the exhaust air/fuel ratio is disposed in an exhaust passage upstream of the catalyst. In operation, the amount of fuel supplied to the engine is increased when the exhaust air/fuel ratio becomes lean, and is reduced when the exhaust air/fuel ratio becomes rich, so that the air/fuel ratio is controlled to around the stoichiometric air/fuel ratio as a central air/fuel ratio. Therefore, even if the air/fuel ratio alternately swings to or fluctuates between the rich side and the lean side, the amounts of NOx, HC and CO can be reduced at the same time.

In this connection, the catalytic conversion efficiency, or the efficiency with which exhaust gas is treated or cleaned by the catalyst, is reduced as the three-way catalyst deteriorates. There is a correlation between the degree of deterioration of the three-way catalyst and the degree of reduction of the O₂ storage function since both of the catalytic conversion and the O₂ storage function involve chemical reaction through the medium of a noble metal. It is thus possible to detect deterioration of the catalyst by detecting reduction of the O₂ storage function. More specifically, it is possible to detect deterioration of the catalyst by measuring the oxygen storage capacity as the maximum amount of oxygen that can be stored in the catalyst in the present state.

In the meantime, a relatively high concentration of a poisoning substance, such as sulfur (S), may be contained in fuel, depending on a geographic area or region of use of the vehicle, for example. Where such a fuel is supplied to the engine, poisoning (e.g., S (sulfur) poisoning) may occur to the catalyst, namely, the poisoning substance may be accumulated in the catalyst, resulting in reduction of the performance of the catalyst. If the poisoning occurs, catalytic reactions to adsorb and release oxygen are impeded or hampered, and the oxygen storage capacity of the catalyst is reduced. In this situation, if another type of fuel having a low concentration of the poisoning substance is re-supplied to the engine, the poisoned condition can be eliminated since the reduction of the performance and deterioration of the catalyst due to poisoning are only temporary conditions. Thus, unless a controller of the engine detects catalyst deterioration while distinguishing from temporary deterioration due to poisoning, there is a possibility of erroneously determining that a catalyst whose exhaust treatment performance is temporarily reduced due to poisoning is deteriorated even though this catalyst should be regarded as being normal. Namely, since the controller for detecting catalyst deterioration is originally intended to detect irrecoverable, permanent deterioration, such as chronological deterioration (deterioration with time), it is necessary to distinguish the deterioration of this type from temporary deterioration due to poisoning.

It is also known that the oxygen storage capacity of the catalyst varies depending on the catalyst temperature. As disclosed in, for example, Japanese Patent Application Publication No. 2003-120266 (JP-A-2003-120266), the rate or degree of change of the oxygen storage capacity with respect to the catalyst temperature is reduced in a similar manner in the case where the catalyst is deteriorated and in the case where the fuel contains a high concentration of sulfur, and it is thus necessary to distinguish these cases from each other.

As is understood from the above description, the oxygen storage capacity of the catalyst varies depending on the catalyst temperature and the presence or absence of poisoning. No suitable system has been found which effectively utilizes this fact when detecting catalyst deterioration, and is able to detect deterioration of a catalyst while distinguishing permanent deterioration from temporary deterioration due to poisoning.

SUMMARY OF THE INVENTION

The present invention has been developed in view of the above situation, and provides a catalyst deterioration detecting system of an internal combustion engine and a catalyst deterioration detecting method, which are able to detect catalyst deterioration while distinguishing permanent deterioration from temporary deterioration due to poisoning.

According to a first aspect of the invention, there is provided a catalyst deterioration detecting system of an internal combustion engine which detects deterioration of a catalyst disposed in an exhaust passage of the internal combustion engine, comprising: a storage capacity measuring unit that measures an oxygen storage capacity of the catalyst, a catalyst temperature detector that detects or estimates a temperature of the catalyst, and a poisoning detector that detects poisoning of the catalyst, based on a relationship between a change in the catalyst temperature detected or estimated by the catalyst temperature detector, and a change in the oxygen storage capacity measured by the storage capacity measuring unit, which change corresponds to the change in the catalyst temperature.

According to a second aspect of the invention, there is provided a catalyst deterioration detecting method of an internal combustion engine for detecting deterioration of a catalyst disposed in an exhaust passage of the internal combustion engine, comprising the steps of: measuring an oxygen storage capacity of the catalyst, detecting or estimating a temperature of the catalyst, and detecting poisoning of the catalyst based on a relationship between a change in the detected or estimated catalyst temperature, and a change in the oxygen storage capacity corresponding to the change in the catalyst temperature.

It is found from experiments and research carried out by the inventors that a change in the catalyst temperature and a change in the oxygen storage capacity corresponding to the change in the catalyst temperature has a certain relationship, namely, the value of the oxygen storage capacity changes to a larger value as the catalyst temperature changes to a higher temperature level. It is also found that the change in the oxygen storage capacity corresponding to the change in the catalyst temperature is relatively large if the catalyst is free from poisoning, and is relatively small if the catalyst suffers poisoning. The catalyst deterioration detecting system and method of the internal combustion engine as described above are able to favorably determine the presence or absence of poisoning of the catalyst by utilizing the relationships as described above. It is thus possible to detect deterioration of the catalyst while distinguishing permanent deterioration to be detected from temporary deterioration due to poisoning of the catalyst.

In the catalyst deterioration detecting system according to the first aspect of the invention, it is preferable that the poisoning detector detects poisoning of the catalyst based on an amount of change of the oxygen storage capacity between at least two points of the catalyst temperature.

In the catalyst deterioration detecting system as described just above, it is preferable that the poisoning detector detects poisoning of the catalyst by comparing a value based on the amount of change of the oxygen storage capacity with a predetermined value, and that the predetermined value is changed in accordance with a value of the oxygen storage capacity measured at a point of the catalyst temperature in a lowest temperature range.

It is found from experiments and research carried out by the inventors that as the present condition of the catalyst is closer to a new product, the value of the oxygen storage capacity measured at a point of the catalyst temperature in the lowest temperature range is larger, and the amount of change of the oxygen storage capacity between at least two points of the catalyst temperature is larger. Thus, the catalyst deterioration detecting system as described above is able to detect poisoning of the catalyst with further improved accuracy by changing the predetermined value in view of the above-described relationships.

According to a third aspect of the invention, there is provided a catalyst deterioration detecting system of an internal combustion engine which detects deterioration of a catalyst disposed in an exhaust passage of the internal combustion engine, comprising: an active air/fuel ratio controller that performs active air/fuel ratio control for causing an air/fuel ratio of an exhaust gas entering the catalyst to be changed to a rich side or a lean side relative to a predetermined central air/fuel ratio, a storage capacity measuring unit that measures an oxygen storage capacity of the catalyst during execution of the active air/fuel ratio control, and a poisoning detector that detects poisoning of the catalyst based on a relationship between a change in a rich-side or lean-side amplitude established during the active air/fuel ratio control, and a change in the oxygen storage capacity measured by the storage capacity measuring unit, which change corresponds to the change in the rich-side or lean-side amplitude.

According to a fourth aspect of the invention, there is provided a catalyst deterioration detecting method of an internal combustion engine for detecting deterioration of a catalyst disposed in an exhaust passage of the internal combustion engine, comprising the steps of: performing active air/fuel ratio control for causing an air/fuel ratio of an exhaust gas entering the catalyst to be changed to a rich side or a lean side relative to a predetermined central air/fuel ratio, measuring an oxygen storage capacity of the catalyst during execution of the active air/fuel ratio control, and detecting poisoning of the catalyst based on a relationship between a change in a rich-side or lean-side amplitude established during the active air/fuel ratio control, and a change in the oxygen storage capacity corresponding to the change in the rich-side or lean-side amplitude.

It is found from experiments and research carried out by the inventors that a change in the rich-side or lean-side amplitude relative to the central air/fuel ratio in the active air/fuel ratio control and a change in the oxygen storage capacity corresponding to the change in the rich-side or lean-side amplitude has a certain relationship, namely, the value of the oxygen storage capacity changes to a larger value as the rich-side or lean-side amplitude changes in an increasing direction (i.e., to a larger value). It is also found that the change in the oxygen storage capacity corresponding to the change in the rich-side or lean-side amplitude is relatively large if the catalyst is free from poisoning, and is relatively small if the catalyst suffers poisoning. Thus, the catalyst deterioration detecting system and method of the internal combustion engine as described above are able to favorably determine the presence or absence of poisoning of the catalyst by utilizing the relationships as described above. It is thus possible to detect deterioration of the catalyst while distinguishing permanent deterioration to be detected from temporary deterioration due to poisoning of the catalyst.

In the catalyst deterioration detecting system according to the third aspect of the invention, it is preferable that the poisoning detector detects poisoning of the catalyst based on an amount of change of the oxygen storage capacity between at least two points of the rich-side or lean-side amplitude.

In the catalyst deterioration detecting system as described just above, it is preferable that the poisoning detector detects poisoning of the catalyst by comparing a value based on the amount of change of the oxygen storage capacity with a predetermined value, and that the predetermined value is changed in accordance with a value of the oxygen storage capacity measured at a point at which the rich-side or lean-side amplitude is at a minimum.

It is found from experiments and research carried out by the inventors that as the present condition of the catalyst is closer to a new product, the value of the oxygen storage capacity measured at a point at which the rich-side or lean-side amplitude is at the minimum is larger, and the amount of change of the oxygen storage capacity between at least two points of the rich-side or lean-side amplitude is larger. Thus, the catalyst deterioration detecting system as described above is able to detect poisoning of the catalyst with further improved accuracy by changing the predetermined value in view of the relationships as described above.

Preferably, the catalyst deterioration detecting system according to the first or third aspect of the invention further includes a permission/inhibition determining unit that permits or inhibits detection of deterioration of the catalyst depending on a result of detection by the poisoning detector.

With the above arrangement, detection of deterioration of the catalyst can be inhibited when it is determined that the catalyst suffers poisoning, thereby preventing an erroneous determination that a catalyst that is being temporarily poisoned is deteriorated permanently.

Preferably, the catalyst deterioration detecting system according to the first or third aspect of the invention further includes a poisoning removal controller that performs poisoning removal control so as to regenerate the catalyst from poisoning when poisoning of the catalyst is detected by the poisoning detector.

With the poisoning removal control thus performed, a temporarily poisoned condition of the catalyst can be eliminated.

The catalyst deterioration detecting system or method according to this invention provides an excellent effect that it can detect permanent deterioration of a catalyst while distinguishing it from temporary deterioration due to poisoning.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing the construction of an internal combustion engine to which catalyst deterioration detecting system and method of the invention is applied;

FIG. 2 is a schematic cross-sectional view showing the structure of a catalyst;

FIG. 3A and FIG. 3B are time charts useful for explaining active air/fuel ratio control;

FIG. 4A and FIG. 4B are time charts similar to those of FIG. 3A and FIG. 3B, which are useful for explaining a method of calculating the oxygen storage capacity;

FIG. 5 is a graph indicating the relationship between the catalyst temperature and the oxygen storage capacity;

FIG. 6 is a flowchart concerning detection of catalyst poisoning according to a first embodiment of the invention;

FIGS. 7A and 7B are flowcharts concerning detection of catalyst poisoning according to a second embodiment of the invention;

FIG. 8 is a graph indicating the relationship between the rich amplitude (or lean amplitude) and the oxygen storage capacity; and

FIG. 9 is a flowchart concerning detection of catalyst poisoning according to a third embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description and the accompanying drawings, the present invention will be described in more detail with reference to exemplary embodiments.

FIG. 1 is a schematic diagram showing the construction of one exemplary embodiment of the invention. As shown in FIG. 1, the internal combustion engine 1 is operable to burn a mixture of fuel and air within a combustion chamber 3 formed in a cylinder block 2 of each cylinder, and cause a piston 4 to reciprocate in the combustion chamber 3 to thereby produce power. The internal combustion engine 1 is a multi-cylinder engine (only one cylinder of which is illustrated in FIG. 1) for a vehicle, more specifically, a spark ignition type gasoline engine.

In a cylinder head of the engine 1, an intake valve Vi for opening and closing an intake port and an exhaust valve Ve for opening and closing an exhaust port are provided for each of the cylinders. The intake valve Vi and exhaust valve Ve for each cylinder are opened and closed by respective camshafts (not shown). An ignition plug 7 operable to ignite the fuel-air mixture in the combustion chamber 3 is mounted in the cylinder head for each cylinder, more specifically, in a portion of the cylinder head which forms the top wall of the combustion chamber 3. An injector (fuel injection valve) 12 is also mounted in the cylinder head for each cylinder, and is operable to inject fuel directly into the combustion chamber 3. The piston 4 is a so-called chambered piston having a recess 4 a formed in the top face thereof. In the engine 1, while air is drawn into the combustion chamber 3, the fuel is directly injected from the injector 12 toward the recess 4 a of the piston 4. As a result, a layer of the fuel-air mixture, which is separated from a surrounding air layer, is formed (i.e., the fuel-air mixture is stratified) in the vicinity of the ignition plug 7, so that stable stratified charge combustion is carried out.

The intake port of each cylinder is connected to a surge tank 8 as an intake-air collective chamber, via a branch pipe for each cylinder, and an intake pipe 13 that forms an intake-air collective passage is connected to the upstream end of the surge tank 8. An air cleaner 9 is provided at the upstream end of the intake pipe 13. An air flow meter 5 for detecting the intake air amount or air mass flow and an electronically controlled throttle valve 10 are mounted in the intake pipe 13 such that the air flow meter 5 is located upstream of the throttle valve 10. The intake port of each cylinder, surge tank 8 and the intake pipe 13 cooperate to form an intake passage.

On the other hand, the exhaust port of each cylinder is connected to an exhaust pipe 6 as an exhaust collective passage, via a branch pipe for each cylinder, and a catalyst 11 in the form of a three-way catalyst having an O₂ storage function is mounted in the exhaust pipe 6. The exhaust port of each cylinder, corresponding branch pipe and the exhaust pipe 6 cooperate to form an exhaust passage. Pre-catalyst sensor and post-catalyst sensor 17, 18 for detecting the exhaust air/fuel ratio are installed on the upstream side and downstream side of the catalyst 11, respectively. The pre-catalyst sensor 17, which is a so-called wide-range air/fuel ratio sensor, is able to continuously detect the air/fuel ratio over a relatively wide range, and outputs a current signal proportional to the detected air/fuel ratio. The post-catalyst sensor 18, which is a so-called O₂ sensor, is characterized in that its output voltage changes sharply when the air/fuel ratio exceeds or falls below the stoichiometric air/fuel ratio.

The above-mentioned ignition plug 7, throttle valve 10, injector 12 and other components are electrically connected to an electronic control unit (hereinafter referred to as “ECU”) 20 that serves as a control device. The ECU 20 includes CPU, ROM, RAM, input and output ports, and a storage device, none of which is illustrated in FIG. 1. Also, the above-mentioned air flow meter 5, pre-catalyst sensor 17, post-catalyst sensor 18, a crank angle sensor 14 for detecting the crank angle of the engine 1, an accelerator position sensor 15 for detecting the accelerator pedal position (or the amount of depression of the accelerator pedal), a throttle opening sensor 22 for detecting the degree of opening of the throttle valve 10, and various other sensors are electrically connected to the ECU 20 via respective A/D converters (not shown), or the like, as shown in FIG. 1. The ECU 20 controls the ignition plug 7, throttle valve 10, injector 12 and other components so as to provide desired outputs, based on the detected values of the above-indicated various sensors, thereby to control the ignition timing, the fuel injection amount, the fuel injection timing and the throttle opening, for example.

The catalyst 11 is adapted to treat NOx, HC and CO at the same time when the air/fuel ratio A/F of exhaust gas entering the catalyst 11 is equal to the stoichiometric air/fuel ratio AFs (which is, for example, 14.6). To enable the catalyst 11 to perform this function, the ECU 20 controls the air/fuel ratio during normal operation of the engine so that a pre-catalyst air/fuel ratio A/Ffr, i.e., an air/fuel ratio of exhaust gas upstream of the catalyst 11, becomes equal to the stoichiometric ratio A/Fs. More specifically, the ECU 20 sets a target air/fuel ratio A/Ft to the stoichiometric ratio A/Fs, and controls the amount of fuel injected from the injector 12 so that the pre-catalyst air/fuel ratio A/Ffr detected by the pre-catalyst sensor 17 becomes equal to the target air/fuel ratio AFt. In this manner, the air/fuel ratio of the exhaust gas entering the catalyst 11 is kept at or near the stoichiometric air/fuel ratio, and the catalyst 11 exhibits the maximum exhaust purification or cleaning performance. As is understood from the foregoing description, the pre-catalyst air/fuel ratio A/Ffr is controlled so as to be equal to the target air/fuel ratio A/Ft all the time.

In the following, the catalyst 11 will be described in more detail. As shown in FIG. 2, the catalyst 11 has a coating material 31 with which a surface of a carrier substrate (not shown) is coated, and catalyst components 32 in the form of particles disposed on the coating material 31. In the catalyst 11, a large number of catalyst components 32 are dispersed on the coating material 31, and are exposed to the interior of the catalyst 11. The catalyst components 32 are formed mainly of a noble metal, such as Pt or Pd, and provide active sites for reaction with exhaust gas components, such as NOx, HC and CO. On the other hand, the coating material 31 serves as a promoter for promoting reactions at the interfaces between the exhaust gas and the catalyst components 32, and contains an oxygen storage component capable of absorbing and releasing oxygen depending on the air/fuel ratio of surrounding gas. The oxygen storage component is, for example, cerium dioxide CeO₂ or zirconia. Where a gas surrounding the catalyst components 32 and the coating material 31 has a richer (i.e., smaller) air/fuel ratio than the stoichiometric air/fuel ratio A/Fs, oxygen that has been stored in the oxygen storage component that exists around the catalyst components 32 is released, whereby unburned components, such as HC and CO, are oxidized by the released oxygen, to be thus converted into harmless substances. Conversely, where a gas surrounding the catalyst components 32 and the coating material 31 has a leaner (i.e., larger) air/fuel ratio than the stoichiometric ratio A/Fs, the oxygen storage component that exists around the catalyst components 32 absorbs oxygen from the surrounding gas, so that NOx is reduced and converted into a harmless substance.

Owing the above-described function of absorbing and releasing oxygen, the catalyst 11 is able to treat three exhaust gas components, i.e., NOx, HC and CO, at the same time even if the pre-catalyst air/fuel ratio A/Ffr varies to some extent relative to the stoichiometric air/fuel ratio A/Fs during normal air/fuel ratio control. In the normal air/fuel ratio control, therefore, it is possible to clean exhaust gas by minutely oscillating the pre-catalyst air/fuel ratio A/Ffr relative to the stoichiometric air/fuel ratio A/Fs as the center of oscillation, and thus causing the catalyst 11 to repeatedly absorb and release oxygen.

When the catalyst 11 is in a condition of a new product, a large number of catalyst components in the form of minute particles are uniformly dispersed on the coating material 31, as described above, and the probability of contact between the exhaust gas and the catalyst components 32 is kept at a high level. As the catalyst 11 deteriorates, however, a part of the catalyst components 32 may disappear, and/or two or more catalyst components 32 may be burned by heat of the exhaust and stuck together into a sintered condition (as indicated by broken lines in FIG. 2). In such cases, the probability of contact between the exhaust gas and the catalyst components 32 may be reduced, which would cause a reduction of the catalytic conversion efficiency. In addition, the amount of the coating material 31 that exists around the catalyst components 32, or the amount of the oxygen storage component, may be reduced, resulting in a reduction of the oxygen storage capability.

As is understood from the above description, there is a correlation between the degree of deterioration of the catalyst 11 and the degree of reduction of the oxygen storage capability possessed by the catalyst 11. In the present embodiment, therefore, the degree of deterioration of the catalyst 11 is measured or determined on the basis of the oxygen storage capability of the catalyst 11. Here, the oxygen storage capability of the catalyst 11 is represented by the magnitude of the oxygen (O₂) storage capacity (OSC; the unit of which is gram) as the maximum amount of oxygen that can be adsorbed and stored by the catalyst 11 in the present condition.

Next, detection of deterioration of the catalyst according to this embodiment will be explained. In this embodiment, the ECU 20 performs active air/fuel ratio control when detecting deterioration of the catalyst 11. The term “active air/fuel ratio control” means control for forcedly changing the pre-catalyst air/fuel ratio A/Ffr to the rich side and the lean side relative to a certain central air/fuel ratio A/Fc. In the following description, the air/fuel ratio which the pre-catalyst air/fuel ratio A/Ffr reaches when it is changed to the rich side will be called “rich air/fuel ratio A/Fr”, and the air/fuel ratio which the pre-catalyst air/fuel ratio A/Ffr reaches when it is changed to the lean side will be called “lean air/fuel ratio A/Fl”. When the pre-catalyst air/fuel ratio A/Ffr is changed to the rich side or the lean side under the active air/fuel ratio control, the oxygen storage capacity OSC of the catalyst 11 is measured.

The detection of deterioration of the catalyst 11 is carried out when the engine 1 is in normal (steady-state) operating conditions, and the catalyst 11 is within an activation temperature range. While the temperature of the catalyst 11 may be directly detected by use of a temperature sensor, the catalyst temperature is estimated based on engine operating conditions in this embodiment. The ECU 20 estimates the temperature of the catalyst 11, using a map or function set in advance through experiments, or the like, based on at least one of the intake air amount GA detected by the air flow meter 5, the engine speed NE calculated based on the output of the crank angle sensor 14, and the engine load KL calculated based on the detected value of the throttle opening sensor 22.

The detection of deterioration of the catalyst 11 is carried out at least once per trip of the internal combustion engine. If deterioration of the catalyst is detected in at least two trips in a row, it is finally determined that the catalyst 11 is in an abnormal or deteriorated condition, and a warning device, such as a check lamp, is activated. Here, one trip means a period from the time at which the engine is started to the time at which the engine is stopped.

In FIG. 3A and FIG. 3B, solid lines indicate the outputs of the pre-catalyst sensor 17 and the post-catalyst sensor 18, respectively, during execution of the active air/fuel ratio control. In FIG. 3A, the broken line indicates a target air/fuel ratio A/Ft generated in the inside of the ECU 20. The output values of the pre-catalyst sensor 17 and post-catalyst sensor 19 are denoted as “pre-catalyst air/fuel ratio A/Ffr” and “post-catalyst air/fuel ratio A/Frr”, respectively.

As shown in FIG. 3A, the target air/fuel ratio A/Ft is forced to be alternately switched between the air/fuel ratio (rich air/fuel ratio A/Fr) that is shifted from the stoichiometric air/fuel ratio A/Fs to the rich side by a certain amplitude (rich amplitude Ar, Ar>0), and the air/fuel ratio (lean air/fuel ratio A/Fl) that is shifted from the stoichiometric ratio A/Fs to the lean side by a certain amplitude (lean amplitude Al, Al>0), relative to the stoichiometric air/fuel ratio as the central air/fuel ratio. Following the switching or oscillation of the target air/fuel ratio A/Ft, the pre-catalyst air/fuel ratio A/Ffr as the actual value switches between the rich air/fuel ratio A/Fr and the lean air/fuel ratio A/Fl with a slight time delay from the target air/fuel ratio A/Ft. It will be understood from this fact that the target air/fuel ratio A/Ft and the pre-catalyst air/fuel ratio A/Ffr are equivalent to each other except that there is a time delay between these ratios.

In the example shown in FIGS. 3A and 3B, the rich amplitude Ar and the lean amplitude Al are equal to each other. For example, the stoichiometric air/fuel ratio A/Fs is equal to 14.6, the rich air/fuel ratio A/Fr is equal to 14.1, the lean air/fuel ratio A/Fl is equal to 15.1, and the rich amplitude Ar, which is equal to the lean amplitude Al, is equal to 0.5. In comparison with the case of normal air/fuel ratio control, the amplitude of the air/fuel ratio is relatively large in the case of the active air/fuel ratio control, namely, the rich amplitude Ar and lean amplitude Al has a relatively large value.

In the meantime, the target air/fuel ratio A/Ft is switched from the lean air/fuel ratio A/Fl to the rich air/fuel ratio A/Fr or vice versa at the time when the output of the post-catalyst sensor 18 switches from rich to leach or from lean to rich. As shown in FIG. 3B, the output voltage of the post-catalyst sensor 18 rapidly changes with reference to the stoichiometric air/fuel ratio A/Fs as a threshold, such that the output voltage becomes equal to or higher than a rich determination value VR when the post-catalyst air/fuel ratio A/Frr is smaller or richer than the stoichiometric air/fuel ratio A/Fs, and the output voltage becomes equal to or lower than a lean determination value VL when the post-catalyst air/fuel ratio A/Frr is larger or leaner than the stoichiometric air/fuel ratio A/Fs. Here, VR is larger than VL. For example, VR is equal to 0.59 (V), and VL is equal to 0.21 (V).

As shown in FIG. 3A and FIG. 3B, when the output voltage of the post-catalyst sensor 18 changes from a rich-side value to the lean side and becomes equal to the lean determination value VL (at time t1), the target air/fuel ratio A/Ft is switched from the lean air/fuel ratio A/Fl to the rich air/fuel ratio A/Fr. Subsequently, when the output voltage of the post-catalyst sensor 18 changes from a lean-side value to the rich side and becomes equal to the rich determination value VR (at time t2), the target air/fuel ratio A/Ft is switched from the rich air/fuel ratio A/Fr to the lean air/fuel ratio A/Fl.

While the active air/fuel ratio control involving the above-described changes in the air/fuel ratio is being carried out, the oxygen storage capacity OSC of the catalyst 11 is measured in the following manner, and deterioration of the catalyst 11 is determined.

Referring to FIG. 3A and FIG. 3B, prior to time t1, the target air/fuel ratio A/Ft is set to the lean air/fuel ratio A/Fl, and lean gas flows into the catalyst 11. During this period (prior to time t1), the catalyst 11 keeps absorbing oxygen, but becomes unable to absorb oxygen any more when the catalyst 11 has absorbed oxygen to the full capacity thereof, thus causing the lean gas to pass through the catalyst 11 and flow down to the downstream side of the catalyst 11. As a result, the post-catalyst air/fuel ratio A/Frr changes to the lean side, and the target air/fuel ratio A/Ft is switched, or reversed, to the rich air/fuel ratio A/Fr at the time (t1) when the output voltage of the post-catalyst sensor 18 reaches the lean determination value VL. Thus, a change in the output of the post-catalyst sensor 18 triggers switching or reversal of the target air/fuel ratio A/Ft.

Following the switching of the target air/fuel ratio A/Ft, rich gas is caused to flow into the catalyst 11, so that the catalyst 11 keeps releasing oxygen stored therein so far. As a result, exhaust gas having approximately the stoichiometric air/fuel ratio A/Fs flows down to the downstream side of the catalyst 11, and the output of the post-catalyst sensor 18 is not reversed since the post-catalyst air/fuel ratio A/Frr does not become rich. As the catalyst 11 continues to release oxygen therefrom, the entire amount of oxygen stored in the catalyst 11 is eventually released from the catalyst 11 after a while. At this point in time, the catalyst 11 becomes unable to release oxygen any more, and the rich gas is passed, through the catalyst 11 and flows down to the downstream of the catalyst 11. As a result, the post-catalyst air/fuel ratio A/Frr changes to the rich side, and the target air/fuel ratio A/Ft is switched to the lean air/fuel ratio A/Fl at the time (t2) when the output voltage of the post-catalyst sensor 18 reaches the rich determination value VR.

The larger the oxygen storage capacity OSC, the longer the period of time for which the catalyst 11 keeps absorbing or releasing oxygen. Namely, the reversal cycle (e.g., period of time between t1 and t2) of the target air/fuel ratio A/Ft is relatively large where the catalyst is not deteriorated, and the reversal cycle of the target air/fuel ratio A/Ft becomes shorter as deterioration of the catalyst progresses.

In view of the fact as described above, the oxygen storage capacity OSC is calculated in the following manner. As shown in FIG. 4A and FIG. 4B, immediately after the target air/fuel ratio A/Ft is switched to the rich air/fuel ratio A/Fr at time t1, the pre-catalyst air/fuel ratio A/Ffr as the actual value is switched to the rich air/fuel ratio A/Fr with a little delay. The oxygen storage capacity dC for each infinitesimal time interval is calculated according to Equation (1) below, from time t11 at which the pre-catalyst air/fuel ratio A/Ffr reaches the stoichiometric air/fuel ratio A/Fs to time t2 at which the target air/fuel ratio A/Ft is reversed next time, and the oxygen storage capacity dC for each infinitesimal time interval is integrated from time t11 to time t2. In this manner, the oxygen storage capacity OSC1 in the oxygen release cycle, i.e., the released oxygen amount, is calculated.

dC=ΔA/F×Q×K=|A/Ffr−A/Fs|×Q×K  (1)

In Equation (1), Q is the fuel injection amount, and K is the proportion of oxygen contained in air, which is equal to about 0.23. The amount of excess air can be calculated by multiplying a difference ΔA/F in the air/fuel ratio by the fuel injection amount Q.

Basically, it may be determined whether the catalyst is deteriorated, by using the oxygen storage capacity OSC1 calculated one time on this occasion. More specifically, the oxygen storage capacity OSC1 thus calculated is compared with a predetermined deterioration determination value, and it is determined that the catalyst 11 is normal if the oxygen storage capacity OSC1 exceeds the deterioration determination value, while it is determined that the catalyst 11 is deteriorated if the oxygen storage capacity OSC1 is equal to or smaller than the deterioration determination value. In this embodiment, however, in order to improve the accuracy, the oxygen storage capacity (an oxygen absorption amount in this case) is also calculated on the lean side in a similar manner, and the calculation is repeated two or more times as needed on the rich side and the lean side. Then, the average of the thus calculated values is compared with the deterioration determination value, so that a final determination on deterioration of the catalyst 11 can be made.

More specifically described with reference to FIG. 4A and FIG. 4B, after the target air/fuel ratio A/Ft is switched to the lean air/fuel ratio A/Fl at time t2, the oxygen storage capacity dC for each infinitesimal time interval is calculated according to the above-indicated Equation (1), and the oxygen storage capacity dC for each infinitesimal time interval is integrated from time t21 at which the pre-catalyst air/fuel ratio A/Ffr reaches the stoichiometric air/fuel ratio A/Fs to time t3 at which the target air/fuel ratio A/Ft is reversed to the rich side next time. In this manner, the oxygen storage capacity OSC2 in the oxygen absorption cycle, i.e., the absorbed oxygen amount, is calculated. The oxygen storage capacity OSC1 of the previous cycle and the oxygen storage capacity OSC2 of this cycle should be substantially equal to each other. In this manner, the oxygen storage capacity OSC is repeatedly calculated to provide a plurality of oxygen storage capacities OSC1, OSC2, . . . OSCn (for example, n≧5), and the average OSCav of these values is compared with a predetermined deterioration determination value OSCs. It is determined that the catalyst 11 is normal if the average value OSCav exceeds the deterioration determination value OSCs, and that the catalyst 11 is deteriorated if the average value OSCav is equal to or smaller than the deterioration determination value OSCs.

The number n of calculation of the oxygen storage capacity OSC may be changed depending upon a parameter or parameters, such as the running distance of the vehicle, which is/are correlated with the progression of catalyst deterioration. For example, the number n is set to a small value when the running distance is relatively small and it is apparently assumed that catalyst deterioration does not progress so much, and the number n is set to a large value when the running distance is relatively large and deterioration may progress to a considerable degree.

In the meantime, if a certain type of fuel containing a relatively high concentration of a poisoning substance, such as sulfur (S), is supplied to the engine, the catalyst is poisoned by the poisoning substance (for example, the catalyst suffers sulfur (S) poisoning), and the catalyst is temporarily brought into a deteriorated condition. In the case of the S (sulfur) poisoning, for example, the catalyst components 32 serving as active sites within the catalyst 11 and the coating material 31 containing the oxygen storage component are covered with sulfate, which may interfere with reaction between the catalyst 11 and exhaust gas. The sulfate can be released or removed from the catalyst 11 if a fuel having a low sulfur concentration is re-supplied to the engine, and the catalyst can recover from sulfur poisoning. It is thus necessary to distinguish such temporary deterioration from permanent deterioration so as to detect catalyst deterioration with a high degree of reliability.

In the present embodiment, therefore, poisoning of the catalyst is detected in the following manner in order to solve the above problem.

FIG. 5 shows some examples of the relationship between the catalyst temperature Tc and the oxygen storage capacity OSC, for use in detection of catalyst poisoning in first and second embodiments of the invention as will be described later. These relationships were obtained through experiments, or the like. In FIG. 5, the solid line indicates the relationship between the catalyst temperature Tc and the oxygen storage capacity OSC with respect to a certain catalyst in the case where the catalyst is free from poisoning, and the broken line indicates the relationship between the catalyst temperature Tc and the oxygen storage capacity OSC with respect to the same catalyst in the case where the catalyst suffers poisoning. As is understood from FIG. 5, in either of the cases, the oxygen storage capacity OSC has a tendency of increasing with an increase in the catalyst temperature Tc. This is because the reacting capability of the catalyst components 32 formed of a noble metal improves as the catalyst temperature Tc becomes higher. On the other hand, the degree of increase of the oxygen storage capability relative to the same increase of the catalyst temperature Tc (i.e., the slope of each graph representing the relationship in FIG. 5) differs between the case where the catalyst suffers poisoning and the case where the catalyst is free from poisoning. More specifically, the degree of increase is reduced in the case where the catalyst suffers poisoning, as compared with the case where the catalyst is free from poisoning. In other words, the relationship between a change in the catalyst temperature Tc and a change in the oxygen storage capacity OSC corresponding to the change in the catalyst temperature Tc differs between the case where the catalyst suffers poisoning and the case where the catalyst is free from poisoning.

In the present embodiment, therefore, the presence or absence of poisoning of the catalyst is determined from the relationship between a change in the catalyst temperature Tc and a change in the oxygen storage capacity OSC corresponding to this change in the catalyst temperature Tc. More specifically, poisoning of the catalyst is detected based on an amount of change of the oxygen storage capacity between at least two points of catalyst temperatures.

Referring to FIG. 5, for example, when the catalyst temperature Tc is equal to a first temperature Ta in a relatively low temperature range, the same oxygen storage capacity OSCa is obtained in the case where the catalyst suffers poisoning and the case where the catalyst is free from poisoning. On the other hand, when the catalyst temperature Tc is equal to a second temperature Tb in a relatively high temperature range, the oxygen storage capacity is equal to OSCb1 in the case where the catalyst is free from poisoning, and is equal to OSCb2 that is smaller than OSCb1 in the case where the catalyst suffers poisoning. Thus, the presence or absence of poisoning can be determined by comparing a value based on the amount of change (OSCb−OSCa) of the oxygen storage capacity between the two points of catalyst temperatures Ta, Tb, with a predetermined value. More specifically, it is determined that the catalyst is free from poisoning when the amount of change (OSCb−OSCa) of the oxygen storage capacity is equal to or larger than the predetermined value, and that the catalyst suffers poisoning when the amount of change (OSCb−OSCa) of the oxygen storage capacity is smaller than the predetermined value.

It is also possible to detect poisoning of the catalyst from values based on the amounts of change of the oxygen storage capacity among three or more points of catalyst temperatures.

Preferably, the above-mentioned predetermined value is changed in accordance with the value of the oxygen storage capacity (OSCa) obtained when the catalyst temperature Tc is at a point (Ta) in the lowest temperature range. It is found from experiments that as the present condition of the catalyst is closer to a new product (with no or small deterioration), the value of the oxygen storage capacity (OSCa) at the above-indicated point (Ta) in the lowest temperature range is larger, and the amount of change (OSCb−OSCa) of the oxygen storage capacity between two points of temperatures is also larger (namely, the slope of each graph of FIG. 5 is steeper). Thus, it is possible to detect catalyst poisoning with further improved accuracy by taking these relationships into consideration.

In the following, detection of catalyst poisoning according to the first embodiment will be explained with reference to FIG. 6. The ECU 20 repeatedly executes the routine of the process illustrated in FIG. 6 at certain time intervals.

Initially, it is determined in step S101 whether preconditions for measurement of the oxygen storage capacity along with execution of the active air/fuel ratio control are satisfied. The preconditions, which are similar to the above-described preconditions for execution of catalyst deterioration detection, are satisfied if the engine 1 is in normal (or steady-state) operating conditions and the catalyst temperature is in a certain activation temperature range. If the conditions are not satisfied, the process of FIG. 6 is finished. If the conditions are satisfied, on the other hand, the ECU 20 proceeds to step S102.

In step S102, an estimated value of the catalyst temperature Tc is acquired. In step S103, it is determined whether the estimated value of the catalyst temperature Tc is within a predetermined first temperature range A. As shown in FIG. 5, the first temperature range A includes the above-mentioned first temperature Ta, and covers a temperature range in the vicinity of the first temperature Ta. In other words, the first temperature range A is a temperature range (Tal<Tc<Tah) between a first lower-limit temperature Tal and a first upper-limit temperature Tah, and the first temperature Ta is included in this temperature range.

If the value of the catalyst temperature Tc is within the first temperature range A, the ECU 20 proceeds to step S106 to measure the oxygen storage capacity at this time, or the first oxygen storage capacity OSCa, and then proceeds to step S107. The first oxygen storage capacity OSCa is preferably the average value of two or more oxygen storage capacities, as described above.

If the value of the catalyst temperature Tc is not within the first temperature range A, on the other hand, the ECU 20 proceeds to step S104 to determine whether the value of the catalyst temperature Tc is within a predetermined second temperature range B. As shown in FIG. 5, the second temperature range B includes the second temperature Tb, and covers a temperature range in the vicinity of the second temperature Tb. In other words, the second temperature range B is a temperature range (Tbl<Tc<Tbh) between a second lower-limit temperature Tbl and a second upper-limit temperature Tbh, and the second temperature Tb is included in this temperature range.

If the value of the catalyst temperature Tc is within the second temperature range B, the ECU 20 proceeds to step S105 to measure the oxygen storage capacity at this time, or the second oxygen storage capacity OSCb, and then proceeds to step S107. It is also preferable to use the average value of two or more oxygen storage capacities as the second oxygen storage capacity OSCb. If the value of the catalyst temperature Tc is not within the second temperature range B, the process of FIG. 6 is finished.

In step S107, it is determined whether measurements of the oxygen storage capacities at the two points of catalyst temperatures, namely, the first oxygen storage capacity OSCa and the second oxygen storage capacity OSCb, have been finished. If the measurements have not been finished, the process of FIG. 6 is finished. If the measurements have been finished, the ECU 20 proceeds to step S108.

In step S108, the rate of change of the oxygen storage capacity ΔOSC as a value based on the amount of change between the first oxygen storage capacity OSCa and the second oxygen storage capacity OSCb is calculated. The rate of change of the oxygen storage capacity ΔOSC is defined by Equation (2) as follows.

ΔOSC=(OSCb−OSCa)/(Tb−Ta)  (2)

As is understood from Equation (2) above, the rate of change of the oxygen storage capacity ΔOSC corresponds to the slope of each graph of FIG. 5. Then, step S109 is executed to compare the rate of change of the oxygen storage capacity ΔOSC with a predetermined value α (see FIG. 5). As described above, the predetermined value α is changed in accordance with the value of the first oxygen storage capacity OSCa measured in step S106 when it is determined that the catalyst temperature Tc is in the first temperature range OSCa. More specifically, the predetermined value α is calculated from a certain map or function, based on the value of the first oxygen storage capacity OSCa measured in step S106, such that the predetermined value α is set to a larger value as the value of the first oxygen storage capacity OSCa increases.

If the rate of change of the oxygen storage capacity ΔOSC is smaller than the predetermined value α, the catalyst is regarded as being poisoned by a poisoning substance, such as sulfur, namely, it is determined that the catalyst suffers poisoning, such as S (sulfur) poisoning. In this case, detection of catalyst deterioration is inhibited in step S110. Thus, the poisoned catalyst is prevented from being erroneously determined as being deteriorated.

If the rate of change of the oxygen storage capacity ΔOSC is equal to or larger than the predetermined value α, on the other hand, the catalyst is regarded as being not poisoned, namely, it is determined that the catalyst is free from poisoning. In this case, detection of catalyst deterioration is permitted in step S111. Subsequently, detection of catalyst deterioration is performed according to another deterioration detection routine. In the manner as described above, the permission or inhibition of the catalyst deterioration detection is determined at the same that the presence or absence of catalyst poisoning is determined.

According to the present embodiment as described above, it is possible to detect temporary deterioration of the catalyst while distinguishing it from permanent deterioration, by determining the presence or absence of poisoning of the catalyst. Since detection of catalyst deterioration is inhibited when catalyst poisoning is detected, the temporarily deteriorated catalyst is prevented from being erroneously recognized as a permanently deteriorated catalyst, and catalyst deterioration is detected with enhanced reliability.

Next, detection of catalyst poisoning according to a second embodiment of the invention will be explained with reference to FIGS. 7A and 7B. The second embodiment is substantially identical with the first embodiment as shown in FIG. 6, namely, steps S201 through S211 of the second embodiment are identical with steps S101 through S111 of the first embodiment. The second embodiment is different from the first embodiment in that step S212 is added subsequent to step S210. In step S212, poisoning removal control (e.g., sulfur discharge control) for regenerating the catalyst from poisoning is carried out. The poisoning removal control is to forcedly control the exhaust air/fuel ratio to a rich-side value that is lower than the stoichiometric air/fuel ratio when the catalyst temperature is a high temperature (e.g., equal to or higher than 650° C.). In this manner, the poisoning substance (such as sulfate) deposited on the catalyst is released or removed from the catalyst, so that the catalyst resumes a non-poisoned or poisoning-free condition.

Next, detection of catalyst poisoning according to a third embodiment of the invention will be explained. FIG. 8 shows some examples of the relationship between the rich amplitude Ar or lean amplitude Al established under the active air/fuel ratio control and the oxygen storage capacity OSC, for use in the third embodiment. These relationships were also obtained through experiments, or the like. Since the relationship between the rich amplitude Ar and the oxygen storage capacity OSC is substantially the same as the relationship between the lean amplitude Al and the oxygen storage capacity OSC, only the relationship between the rich amplitude Ar and the oxygen storage capacity OSC will be explained for the sake of brevity.

In FIG. 8, the solid line indicates the relationship between the rich amplitude Ar and the oxygen storage capacity OSC with respect to a certain catalyst in the case where the catalyst is free from poisoning, and the broken line indicates the relationship between the rich amplitude Ar and the oxygen storage capacity OSC with respect to the same catalyst in the case where the catalyst suffers poisoning. As is understood from FIG. 8, the relationship of FIG. 8 in each case is similar to the relationship between the catalyst temperature Tc and the oxygen storage capacity OSC as shown in FIG. 5.

In either of the cases as described above, the oxygen storage capacity OSC has a tendency of increasing with an increase in the rich amplitude Ar. The reason of this tendency is that as the rich amplitude Ar increases, a portion of the catalyst 11 that can be utilized for absorption or release of oxygen extends from the surface of the coating material 31 to a deeper position and to a further downstream position. It is, however, to be noted that the degree of increase of the oxygen storage capacity OSC relative to the same increase of the rich amplitude Ar (i.e., the slope of each graph of FIG. 8) differs between the case where the catalyst suffers poisoning and the case where the catalyst is free from poisoning, and that the degree of increase of the oxygen storage capacity OSC is reduced when the catalyst suffers poisoning. Namely, the relationship between a change in the rich amplitude Ar and a change in the oxygen storage capacity OSC corresponding to the change in the rich amplitude Ar differs between the case where the catalyst suffers poisoning and the case where the catalyst is free from poisoning.

In the third embodiment, therefore, poisoning of the catalyst is detected from the relationship between a change in the rich amplitude Ar and a change in the oxygen storage capacity OSC corresponding to the change in the rich amplitude Ar. More specifically, poisoning of the catalyst is detected based on the amount of change of the oxygen storage capacity between at least two points of rich amplitudes Ar.

Referring to FIG. 8, for example, when the rich amplitude Ar is equal to a relatively small first amplitude Ara, the same oxygen storage capacity OSCa is obtained in the case where the catalyst suffer poisoning and the case where the catalyst is free from poisoning. On the other hand, when the rich amplitude Ar is equal to a relatively large second amplitude Arb, the oxygen storage capacity is equal to OSCb1 where the catalyst is free from poisoning, while it is equal to OSCb2 that is smaller than OSCb1 where the catalyst suffers poisoning. Thus, the presence or absence of poisoning can be determined by comparing a value based on the amount of change (OSCb−OSCa) of the oxygen storage capacity between the two points of rich amplitudes Ara, Arb, with a predetermined value. More specifically, it is determined that the catalyst is free from poisoning when the value based on the amount of change (OSCb−OSCa) of the oxygen storage capacity is equal to or larger than the predetermined value, and it is determined that the catalyst suffers poisoning when the value based on the amount of change (OSCb−OSCa) of the oxygen storage capacity is smaller than the predetermined value.

It is also possible to detect poisoning of the catalyst based on values based on the amounts of change of the oxygen storage capacity among three or more points of rich amplitudes.

Preferably, the above-mentioned predetermined value is changed in accordance with the value of the oxygen storage capacity (OSCa) obtained when the rich amplitude Ar is at the smallest point (Ara in FIG. 8). It is found from experiments that as the present condition of the catalyst is closer to a new product with no or small deterioration, the value of the oxygen storage capacity (OSCa) obtained when the rich amplitude Ar is at the smallest point (Ara) is larger, and the amount of change (OSCb−OSCa) of the oxygen storage capacity between two points of rich amplitudes is also larger (namely, the slope of each graph of FIG. 8 is steeper). Thus, it is possible to detect catalyst poisoning with further improved accuracy by taking these relationships into consideration.

The above description with regard to the relationship between the rich amplitude Ar and the oxygen storage capacity OSC is also applied to the relationship between the lean amplitude Al and the oxygen storage capacity OSC.

The detection of catalyst poisoning according to the third embodiment will be more specifically explained with reference to FIG. 9. The ECU 20 repeatedly executes the routine of the process as shown in FIG. 9 at predetermined time intervals.

Initially, it is determined in step S301 whether preconditions for measurement of the oxygen storage capacity are satisfied, as in the above-indicated step S101 of FIG. 6. If the conditions are not satisfied, the process of FIG. 9 is finished. If the conditions are satisfied, on the other hand, the ECU 20 proceeds to step S302.

In step S302, active air/fuel ratio control is performed with the first rich amplitude Ara and first lean amplitude Ala (Ara=Ala) that are relatively small rich amplitude and lean amplitude. In step S303, the oxygen storage capacity corresponding to the first rich amplitude Ara and first lean amplitude Ala, or the first oxygen storage capacity OSCa, is measured. The first oxygen storage capacity OSCa is preferably the average value of two or more oxygen storage capacities, as described above.

Subsequently, in step S304, the rich amplitude and the lean amplitude are changed, and active air/fuel ratio is performed with the second rich amplitude Arb and second lean amplitude Alb (Arb=Alb) that are relatively large rich amplitude and lean amplitude. The second rich amplitude Arb and second lean amplitude Alb established at this time are larger than the first rich amplitude Ara and first lean amplitude Ala by, for example, about 0.5-1 in terms of the air/fuel ratio. In step S305, the oxygen storage capacity corresponding to the second rich amplitude Arb and second lean amplitude Alb, or the second oxygen storage capacity OSCb, is measured. It is also preferable to use the average value of two or more oxygen storage capacities as the second oxygen storage capacity OSCb.

In step S306, the rate of change of the oxygen storage capacity ΔOSC as a value based on the amount of change between the first oxygen storage capacity OSCa and the second oxygen storage capacity OSCb is calculated. The rate of change of the oxygen storage capacity ΔOSC is defined by Equation (3) as follows.

ΔOSC=(OSCb−OSCa)/(Arb−Ara)  (3)

The denominator in the above Equation (3) may be (Alb−Ala). As is understood from Equation (3) above, the rate of change of the oxygen storage capacity ΔOSC corresponds to the slope of each graph of FIG. 8. Then, step S307 is executed to compare the rate of change of the oxygen storage capacity ΔOSC with a predetermined value β (see FIG. 8). The predetermined value β is changed in accordance with the value of the first oxygen storage capacity OSCa measured in step S303. More specifically, the predetermined value β is calculated from a certain map or function, based on the value of the first oxygen storage capacity OSCa measured in step S303, such that the predetermined value β is set to a larger value as the value of the first oxygen storage capacity OSCa increases.

If the rate of change of the oxygen storage capacity ΔOSC is smaller than the predetermined value β, the catalyst is regarded as being poisoned by a poisoning substance, such as sulfur, namely, poisoning of the catalyst is detected. In this case, detection of catalyst deterioration is inhibited in step S308, and catalyst poisoning removal control (e.g., sulfur discharge control) is carried out in step S309 similar to the above-indicated step S212.

If the rate of change of the oxygen storage capacity ΔOSC is equal to or larger than the predetermined value β, on the other hand, the catalyst is regarded as being not poisoned, namely, it is determined that the catalyst is free from poisoning. In this case, detection of catalyst deterioration is permitted in step S310.

According to the third embodiment as described above, it is possible to favorably detect poisoning of the catalyst in a manner similar to those of the first and second embodiments, by utilizing the relationship between a change in the amplitude of the air/fuel ratio during active air/fuel ratio control and a change in the oxygen storage capacity. Thus, the third embodiment provides effects or advantages similar to those of the first and second embodiments.

While some embodiments of the invention have been described in detail, various other embodiments of the invention may be considered. While the internal combustion engine is of a direct injection type in the illustrated embodiments, this invention is also applicable to internal combustion engines of other fuel injection types, such as an intake port (intake passage) injection type or a dual injection type serving as both the direct injection type and the port injection type. While a so-called O₂ sensor is used as the post-catalyst sensor 18 in the illustrated embodiments, an air/fuel ratio sensor similar to the pre-catalyst sensor 17 may be used as the post-catalyst sensor 18. It is also possible to use a so-called O₂ sensor as the pre-catalyst sensor 17.

In the illustrated embodiments, the ECU 20 provides at least a part or parts of the storage capacity measuring unit, catalyst temperature detector, poisoning detector, active air/fuel ratio controller, permission/inhibition determining unit, and the poisoning removal controller as mentioned in “SUMMARY OF INVENTION” in connection with the invention.

While the invention has been described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the exemplary embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the exemplary embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention. 

1. A catalyst deterioration detecting system of an internal combustion engine which detects deterioration of a catalyst disposed in an exhaust passage of the internal combustion engine, comprising: a storage capacity measuring unit that measures an oxygen storage capacity of the catalyst; a catalyst temperature detector that detects or estimates a temperature of the catalyst; and a poisoning detector that detects poisoning of the catalyst, based on a relationship between a change in the catalyst temperature detected or estimated by the catalyst temperature detector, and a change in the oxygen storage capacity measured by the storage capacity measuring unit, which change corresponds to the change in the catalyst temperature.
 2. The catalyst deterioration detecting system according to claim 1, wherein the poisoning detector detects poisoning of the catalyst based on an amount of change of the oxygen storage capacity between at least two points of the catalyst temperature.
 3. The catalyst deterioration detecting system according to claim 2, wherein the poisoning detector detects poisoning of the catalyst by comparing a value based on the amount of change of the oxygen storage capacity with a predetermined value, and the predetermined value is changed in accordance with a value of the oxygen storage capacity measured at a point of the catalyst temperature in a lowest temperature range.
 4. The catalyst deterioration detecting system according to claim 1, further comprising a permission/inhibition determining unit that permits or inhibits detection of deterioration of the catalyst depending on a result of detection by the poisoning detector.
 5. The catalyst deterioration detecting system according to claim 1, further comprising a poisoning removal controller that performs poisoning removal control so as to regenerate the catalyst from poisoning when poisoning of the catalyst is detected by the poisoning detector.
 6. A catalyst deterioration detecting system of an internal combustion engine which detects deterioration of a catalyst disposed in an exhaust passage of the internal combustion engine, comprising: an active air/fuel ratio controller that performs active air/fuel ratio control for causing an air/fuel ratio of an exhaust gas entering the catalyst to be changed to a rich side or a lean side relative to a predetermined central air/fuel ratio; a storage capacity measuring unit that measures an oxygen storage capacity of the catalyst during execution of the active air/fuel ratio control; and a poisoning detector that detects poisoning of the catalyst based on a relationship between a change in a rich-side or lean-side amplitude established during the active air/fuel ratio control, and a change in the oxygen storage capacity measured by the storage capacity measuring unit, which change corresponds to the change in the rich-side or lean-side amplitude.
 7. The catalyst deterioration detecting system according to claim 6, wherein the poisoning detector detects poisoning of the catalyst based on an amount of change of the oxygen storage capacity between at least two points of the rich-side or lean-side amplitude.
 8. The catalyst deterioration detecting system according to claim 7, wherein the poisoning detector detects poisoning of the catalyst by comparing a value based on the amount of change of the oxygen storage capacity with a predetermined value, and the predetermined value is changed in accordance with a value of the oxygen storage capacity measured at a point at which the rich-side or lean-side amplitude is at a minimum.
 9. The catalyst deterioration detecting system according to claim 6, further comprising a permission/inhibition determining unit that permits or inhibits detection of deterioration of the catalyst depending on a result of detection by the poisoning detector.
 10. The catalyst deterioration detecting system according to claim 6, further comprising a poisoning removal controller that performs poisoning removal control so as to regenerate the catalyst from poisoning when poisoning of the catalyst is detected by the poisoning detector.
 11. A catalyst deterioration detecting method of an internal combustion engine for detecting deterioration of a catalyst disposed in an exhaust passage of the internal combustion engine, comprising: measuring an oxygen storage capacity of the catalyst; detecting or estimating a temperature of the catalyst; and detecting poisoning of the catalyst based on a relationship between a change in the detected or estimated catalyst temperature, and a change in the oxygen storage capacity corresponding to the change in the catalyst temperature.
 12. A catalyst deterioration detecting method of an internal combustion engine for detecting deterioration of a catalyst disposed in an exhaust passage of the internal combustion engine, comprising: performing active air/fuel ratio control for causing an air/fuel ratio of an exhaust gas entering the catalyst to be changed to a rich side or a lean side relative to a predetermined central air/fuel ratio; measuring an oxygen storage capacity of the catalyst during execution of the active air/fuel ratio control; and detecting poisoning of the catalyst based on a relationship between a change in a rich-side or lean-side amplitude established during the active air/fuel ratio control, and a change in the oxygen storage capacity corresponding to the change in the rich-side or lean-side amplitude. 